1. Field of the Invention
The invention relates to electromagnetic interference (EMI) filters, and particularly to an EMI filter suitable for filtering common mode noise and differential mode noise.
2. Brief Discussion of the Related Art
Presently, electronic devices are frequently used in daily life, and power is required for operation thereof. When alternating current (AC) power is employed, parasitic capacitance or stray capacitance may occur in the power supply or the high-frequency converter, and noise of stray current in the AC current, leading to electromagnetic interference (EMI).
Generally, noise in the AC current comprises differential mode noise and common mode noise. There are several conventional circuit compositions for filtering the noise. One conventional circuit composition is shown in FIG. 1, in which an AC power supply 10 is connected to a first differential mode EMI filter 11 and a second differential mode EMI filter 12, and the two differential mode EMI filters are electrically connected to the two input terminals of a common mode EMI filter 13 respectively. The two input terminals of the common mode EMI filter 13 are electrically connected to two terminals of a loading 14 respectively, and the loading 14 has a grounding terminal 15 to guide excess current to the ground. The first differential mode EMI filter 11 and the second differential mode EMI filter 12 are employed to filter differential mode noise. Referring to FIG. 3, the first differential mode EMI filter 11 and the second differential mode EMI filter 12 are respectively formed by a column-shaped core wound with a wire 111. The current flows from an input terminal 1111 of the wire 111, circles the core 11 repeatedly and leaves through an output terminal 1112. With magnetic induction of the core 11, differential mode noise is filtered. The common mode EMI filter 13 filters common mode noise. Referring to FIG. 2, the common mode EMI filter 13 is formed by a ring-shaped core 130 wound with first wire 131 and second wire 132. The AC power supply is electrically connected to the first input terminal 1311 and the second input terminal 1321. Current from the first input terminal 1311 encircles a portion of the ring-shaped core 130 and is output through the first output terminal 1312, and current from the second input terminal 1321 encircles another portion of the ring-shaped core 130 and is output through the second output terminal 1322. The first wire 131 and the second wire 132 are wound in the same direction. Thus, the common mode EMI filter 13 does not perform any induction for the differential mode noise due to neutralized magnetic induction, but the flux generated for the common mode noise is doubled, and the common mode noise of the AC current is filtered by the magnetic induction of the ring-shaped core 130. With the above-mentioned circuit, common mode noise and differential mode noise are filtered, and the first differential mode EMI filter 11, the second differential mode EMI filter 12 and the common mode EMI filter 13 can be alternatively disposed without limited thereto. For example, the AC power supply electrically connected to the common mode EMI filter 13 and to the first differential mode EMI filter 11 and the second differential mode EMI filter 12.
In the above-mentioned circuit composition, however, two differential mode EMI filters and one common mode EMI filter are required to filter common mode noise and differential mode noise. As a result, cost of the electrical device is increased, and manufacturing thereof complicated. There is a need to provide an element to filter common mode noise and differential mode noise.